Code compressor for multilevel interleaved multiplexed noise codes

ABSTRACT

Multilevel code mates A and B expanded in accordance with the expression  a)I (b K ) and B=(a K )I (b) are compressed to a lobeless basic mate pair by repeatedly compressing corresponding code mate pairs in successive stages to provide diminishing codes of half the number of code bits by amplifying the code mates by a predetermined amplification factor and then adding and subtracting the code mates in accordance with expressions A i-1  =A i  +B i   K .sbsp.i and B i-1  =A i   K .sbsp.i -B i  where i is the i th  compression stage and K i  is the amplification factor applied to the codes A i  and B i .

The invention described herein may be manufactured, used and licensed byor for the Government for governmental purposes without the payment ofany royalties thereon or therefor.

CROSS REFERENCE TO RELATED APPLICATIONS

This invention is related to the following co-pending applications filedin the name of the present inventor:

U.S. Ser. No. 502,416, now abandoned, entitled, "Code Compressor ForInterleaved Multiplexed Noise Codes", filed in the name of the presentinventor on June 8, 1983;

U.S. Ser. No. 456,157, now U.S. Pat. No. 4,471,342, entitled, "PulseCompressor For Multiplexed Noise Codes", filed in the name of thepresent inventor on Jan. 6, 1983;

U.S. Ser. No. 533,183, entitled, "Multilevel Noise Code Mate PairGeneration And Utilization Of Such Codes", filed in the name of thepresent inventor on Sept. 19, 1983;

U.S. Ser. No. 550,853, entitled, "Code Generator For MultilevelInterleaved Multiplexed Noise Codes", filed in the name of the presentinventor on Nov. 14, 1983; and

U.S. Ser. No. 551,431, entitled, "Multilevel Mate Pair Code CompressorFor Codes Expanded By The Process Of Butting", filed in the name of thepresent inventor on Nov. 14, 1983.

FIELD OF THE INVENTION

This invention relates generally to the compression of noise codeshaving autocorrelation functions which upon detection with a matchedfilter provides an impulse and more particularly to the compression ofsuch codes comprised of expanded multilevel, multiplexed noise code matepairs generated by the process of interleaving.

BACKGROUND OF THE INVENTION

Radio communications systems utilizing multiplexed noise codes aregenerally known. A typical example is shown and described in U.S. Pat.No. 4,293,953, entitled, "Bi-Orthogonal PCM Communications SystemEmploying Multiplexed Noise Codes," which issued to Frank S. Gutleber,the present inventor, on Oct. 6, 1981.

The concept of code expansion and compression for the general class ofmultiplexed noise codes comprised of noise code mate pairs havingautocorrelation functions which upon detection in a matched filterprovide an impulse is also generally known. Such concepts are disclosed,for example, in U.S. Pat. No. 3,461,451, entitled, "Code Generator ToProduce Permutations Of Code Mates", which issued to the presentinventor on Aug. 12, 1969; U.S. Pat. No. 3,519,746, entitled, "Means AndMethods To Obtain An Impulse Autocorrelation Function" which issued tothe present inventor on July 7, 1970; and U.S. Pat. No. 3,634,765,entitled, "System To Provide An Impulse Autocorrelation Function . . . "which issued to the present inventor on Jan. 11, 1972.

In the above mentioned related application, U.S. Ser. No. 502,416, nowabandoned, entitled, "Code Compressor For Interleaved Multiplexed NoiseCodes", there is disclosed the concept of repetitively compressingcorresponding code mate pairs in successive stages to provide everdiminishing codes of half the bit quantity and twice the amplitude untila single pulse results at the location of the final code bit whoseamplitude is increased by a factor of twice the number of bits in eachof the original interleaved codes. The apparatus employed comprises aseries of cascaded code separators separated by delay lines providingever increasing delays which are multiples of the pulsewidth of eachcode bit and wherein the delay is doubled at each stage until the laststage is reached and a delay of one half the original code length of theinterleaved codes is provided, whereupon a combining of the codesresults in an impulse autocorrelation function.

In the above referenced related application U.S. Ser. No. 551,431,entitled, "Multilevel Mate Pair Code Compressor For Codes Expanded ByThe Process Of Butting", there is disclosed the concept of compressingmultilevel, multiplexed code mate pairs by repeatedly compressingcorresponding code mate pairs in successive stages to providediminishing codes of half the code length by amplifying the code matesby a predetermined amplification factor and then adding and subtractingthe code mates in accordance with the following general rule:

    A.sub.i-1 =A.sub.i +B.sub.i.sup.K.sbsp.i

and

    B.sub.i-1 =A.sub.i.sup.K.sbsp.i -B.sub.i

where i is the i_(th) compression stage and the exponent K_(i) is theamplification factor for the i_(th) stage. During the compressionprocess, however, an ever decreasing time delay equal to half the inputcode width is provided so that the respective code mates generated bybutting line up with one another in a time coincident relationship.

Accordingly, it is an object of the present invention to provide animprovement in the compression of noise codes.

Another object of the present invention is to provide an improvement incompression of multiplexed code mate pairs having more than oneamplitude level.

Still another object of the invention is to provide an improvement inthe compression of multilevel, multiplexed code mate pairs generated bythe process of interleaving code mates.

SUMMARY

These and other objects are achieved by compressing multilevel,multiplexed code mate pairs that result from the expansion of code matesusing a code interleaving process which utilizes the repeatedapplication of an expansion rule forming expanded Codes A and B inaccordance with the following expressions: A=(a)I(b^(K)), andB=(a^(K))I(b) where the component K represents the amplification factorof the code bits of codes a and b, b represents the complement ornegative of code b, and the I signifies that code b is interleaved withcode a, that is, (a)I(b)=a₁, b₁, a₂, b₂, . . . a_(n), b_(n).Furthermore, in the expansion processs, any of the four sub-elementsmaking up the expanded code may be inverted or made negative and theamplified segments can, when desired, be transposed.

The code mates thus expanded are compressed in this invention to alobeless basic mate pair by repeatedly compressing corresponding codemate pairs in successive stages to provide diminishing codes of one halfof the number of code bits included in the applied code mate pair byamplifying the code mates by a predetermined amplification factor,lining up the code mates in time coincidence, and then adding andsubtracting the code mates in accordance with the following generalrule:

    A.sub.i-1 =A.sub.i +B.sub.i.sup.K.sbsp.i

and

    B.sub.i-1 =A.sub.i.sup.K.sbsp.i -B.sub.i

where i is the i_(th) compression stage, and the exponent K_(i) is theamplification factor for the i_(th) stage and furthermore, an everincreasing time delay equal to a multiple of the code bit width isprovided, which doubles at each stage so that the code mates at eachcompression stage line up in time coincidence with one another.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram illustrative of transmitterapparatus for a noise modulated communications system utilizing a pairof expanded multilevel code mates generated by the process ofinterleaving;

FIG. 2 is a functional block diagram of receiver apparatus of a noisemodulated communication system for receiving and compressing theexpanded multilevel codes in accordance with the principles of theinvention;

FIG. 3 is a diagram illustrative of an embodiment of the code compressorshown in FIG. 2; and

FIG. 4 is a time related diagram of the interleaved code compressionprocess provided by the compressor shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to digital codes referred to as noisecodes, meaning that the information is coded with a code that is "noiselike" and that it will compress to an impulse when detected with amatched filter. In particular, one class of noise codes are knownwherein pairs of coded signals termed "code mates" have autocorrelationfunctions which provide a peak output at a given time and a zero outputor outputs having the same magnitude but opposite polarity, at all othertimes. When code mate signals, for example, are multiplexed, matchedfilter detected and linearly added, there is provided a lobeless impulseoutput of relatively high amplitude at one given time (τ=0) and zerooutput at all other times (τ≠0). Mathematically, for a pair of codemates a and b,

    .0..sub.a (τ)=-.0..sub.b (τ)                       (1)

for all τ≠0, where .0._(a) (τ) is the autocorrelation function of codea, .0._(b) (τ) is the autocorrelation function of code b and wherein τ=0is the location of the main lobe.

In the above referenced related application Ser. No. 550,853, entitled,"Code Generator For Multilevel Interleaved Multiplexed Noise Codes",there is disclosed a code expansion process which results in multilevelcode mate pairs which meet the requirement of equation (1) while beinggenerated by the repeated application of the following general rule:

    A=(a)I(b.sup.K)                                            (2)

    B=(a.sup.K)I(b)                                            (3)

where the exponent K signifies an amplitude gain factor of the code bitsa₁, a₂ . . . a_(n) in code a and b₁, b₂ . . . b_(n) in code b, brepresents the complement or negative of code b and wherein I signifiesthat the two codes a and b are interleaved, i.e., (a)I(b)=a₁, b₁, a₂,b₂, . . . a_(n), b_(n). Further in the expansion process signified byequations (2) and (3), any of the four sub-elements making up theexpanded code could be made negative, and/or the amplified segments maybe transposed.

Returning attention now to the present invention, consider for purposesof illustration a multilevel code mate pair A₃ and B₃ which have beengenerated in accordance with the expansion rule expressed in equations(2) and (3). The expansion process, furthermore, can be illustrated inthe following example. Letting a and b represent the basic code matepair, using the aforementioned expansion rule results in A₁ and B₁ beingdeveloped as:

    A.sub.1 =(a)I(b.sup.K.sbsp.1)                              (4)

    B.sub.1 =(a.sup.K.sbsp.1)I(b)                              (5)

Expanding codes A₁ and B₁ yields codes A₂ and B₂ which can be expressedas:

    A.sub.2 =(A.sub.1)I(B.sub.1.sup.K.sbsp.2)                  (6)

    B.sub.2 =(A.sub.1.sup.K.sbsp.2)I(B.sub.1)                  (7)

The expansion process can be repeated any number of times in psuccessive stages where in general each expansion stage can be definedby the following general expressions:

    A.sub.i+1 =(A.sub.i)I(B.sub.i.sup.K.sbsp.i)                (8)

    B.sub.i+1 =(A.sub.i.sup.K.sbsp.i)I(B.sub.i)                (9)

Rather than compressing the two expanded codes A_(n) and B_(n) in aconventional matched filter, the present invention compresses the codescollectively rather than individually. An inspection of the two codes,for example, A_(n) =A₃ and B_(n) =B₃ as shown in FIG. 4 reveals thatcompression of these codes results when the following mathematicaloperations are performed, resulting in amplified versions of compressedcodes A₂ and B₂ being generated as code mates in the following manner:

    A.sub.2 '=A.sub.3 +B.sub.3.sup.K.sbsp.3                    (10)

    B.sub.2 '=A.sub.3.sup.K.sbsp.3 -B.sub.3                    (11)

Utilizing the above relationships results in codes A₂ ' and B₂ ' in FIG.4 being expressed as: ##EQU1##

It can be shown, for example, by reference to related application Ser.No. 550,853 that A₂ ' and B₂ ' are equal to A₂ and B₂ multiplied by (K₃² +1). An inspection of those two compressed codes reveals that the samegeneral operation can provide an amplified version of further compressedcode mates A₁ and B₁, resulting in codes A₁ ' and B₁ ' being generatedwhen utilizing a multiplying factor K₂ and lining up the two codes byapplying a time delay of one code bit width (τ). These codes can beexpressed as:

    A.sub.1 '=A.sub.2 +B.sub.2.sup.K.sbsp.2                    (14)

    B.sub.1 '=A.sub.2.sup.K.sbsp.2 -B.sub.2                    (15)

Performing these operations results in: ##EQU2##

Applying these compression equations yields: ##EQU3##

It can be seen from equations (20) and (21) that the multilevel matepairs A₃ and B₃ have been compressed down to a basic mate pair a and bamplified by the factor (K₁ ² +1) (K₂ ² +1)(K₃ ² +1) which correspondsto the coherent summation of the individual bits and is equal to thatwhich would result from a matched filter detector i.e. .0._(A).sbsb.3(τ) and .0._(B).sbsb.3 (τ), in accordance with the known prior art.Further, by delaying code mate A_(o) ' by a time delay of 4τ, i.e.doubling that of the previous stage, codes A_(o) ' and B_(o) ' line upwhereupon they can be linearly added to further compress the basic matecodes a and b. Since a and b form a mate pair, they can be compresseddown to a single impulse autocorrelation function using appropriatecompression stages. For example, if a=a₁ . . . a₂ =1 . . . 0 and b=b₁ .. . b₂ =0 . . . 0, then A_(o) ' and B_(o) ' can be compressed to asingle impulse as shown below.

Adding A_(o) 'd and B_(o) ' yields ##EQU4##

Subtracting (22) from (23) results in ##EQU5##

And delaying the output given by (27) by 8τ lines up the two impulsesignals [(24) and (27)] and adding them then provides a final singleimpulse output equal to 0^(4x) where x=(K₁ ² +1)(K₂ ² +1)(K₃ ² +1).

Thus what has been shown and described is a compression process whereincodes generated from the general expansion rule set forth in equations(10) and (11) are coherently compressed to a lobeless basic mate pair byrepeatedly applying the following general operation:

    A.sub.i-1 =A.sub.i +B.sub.i.sup.K.sbsp.i                   (28)

    B.sub.i-1 =A.sub.i.sup.K.sbsp.i -B.sub.i                   (29)

This process is similar to that utilized for compression multilevelbutted code mates as disclosed in related application U.S. Ser. No.551,431 except that now the time delay between compression stages issuccessively doubled whereas in the former case it is successivelydecreased in order to provide time coincidence between code mates.

While a functional block diagram of a code compressor which is operableto perform the operations identified above is shown in FIG. 3, a noisemodulated pulse communications system employing such codes is typicallydisclosed in FIGS. 1 and 2.

Referring first to FIG. 1, reference numeral 10 denotes a basic codemate generator for generating code mates a and b and which are utilizedto generate multilevel expanded codes A and B in accordance with theforegoing description. The codes a and b are fed out in a time relatedmulti-bit binary digital sequence to code expander apparatus designatedby reference numeral 12 and which are generated, for example, inaccordance with the expansion process disclosed in related application,U.S. Ser. No. 550,853, entitled, "Code Generator For MultilevelInterleaved Multiplexed Noise Codes". The expanded multilevel codes Aand B are applied to respective modulator circuits 14 and 16 whichadditionally have applied thereto a carrier frequency generated by acarrier frequency generator 18. The output of the modulators 14 and 16comprise, for example, separate bi-phase modulated codes which are fedto a multiplexer 20 which operate to either time or frequency multiplexthe carrier modulated signals A and B. The output of the multiplexer 20is fed to an RF amplifier 22 where an RF carrier containing themultiplexed codes A and B are radiated from an antenna 24.

With respect to FIG. 2, there is disclosed receiver apparatus which isresponsive to the RF signal radiated from the antenna 24 and containingthe codes A and B. Accordingly, the radio receiver apparatus is showncoupled to a receiving antenna 28 which is operable to translate themultiplexed codes A and B at the RF frequency to an IF frequencywhereupon they are fed to a demultiplexer 30. The demultiplexer isoperable to output the expanded multilevel codes A and B separatelywhere they are applied to a code compressor 32, the details of which areshown in FIG. 3 and which is operable to generate the basic mate paircode outputs of A_(o) ' and B_(o) ' to provide respectiveautocorrelation function outputs .0._(A).sbsb.3 (τ) and .0._(B).sbsb.3(τ). The autocorrelation function outputs of the compressor 32 can thenbe applied to appropriate compression stages that are operable todevelop a substantially lobeless single impulse output signal .0._(T)(τ).

Referring now to FIG. 3, there is disclosed a functional block diagramof the compressor 32 shown in FIG. 2 where A=A₃ and B=B₃. The compressoras shown in FIG. 3 is comprised of three compressor stages 36₁, 36₂ and36₃, each of which separately implements the compression process definedby the general equations (22) and (23). The first expansion stage 36₁implements the mathematical operations set forth in equations (10) and(11) whereupon the input multilevel code mates A₃ and B₃ are compressedinto the compressed codes A₂ ' and B₂ '. As shown, input code B₃ is fedto an amplification stage comprised of a pulse amplifier 38₁ where it isamplified by the gain factor K₃. The output of the amplifier 38₁ is fedto a first linear adder 40₁ which also is coupled to the other inputcode A₃. The output of the adder 40₁ comprises a compressed code A₂ ' inaccordance with equation (10). The code mate of A₂ ' is obtained byfeeding the code B₃ to a signal inverter 42₁ which is operable to changethe polarity of a positive pulse to a negative pulse and vice versa, andthus outputs the complement or negative of code B₃, i.e. B₃. The codeoutput from the inverter 42₁ is fed to a second linear adder 44₁ alongwith the code A₃ which has been multiplied by the gain factor K₃ in theamplifier 46₁. Since an algebraic subtraction can be obtained bychanging the sign of the subtrahend and thereafter performing anaddition, the adder 44₁ provides an output of code B₂ ' as defined inequation (11). As shown in FIG. 4, the compressed code B₂ ' is actuallydelayed in time relative to the code A₂ ' by one code bit slot width (τ)which is the time duration of one code bit a_(i) or b_(i). It becomesnecessary, therefore, to time delay the compressed code A₂ ' by a time τto line it up with the delayed code B₂ ' prior to compressing the codesin the next or succeeding compression stage 36₂. This is provided by thetime delay device 48₁ which typically comprises a delay line, a wellknown piece of apparatus.

Referring now to the second compression stage 36₂, the code B₂ ' isamplified by the gain factor K₂ in the amplifier 38₂ and added to thedelayed code A₂ 'd in the linear adder 40₂ providing a double compressedoutput code of A₁ '. The code mate to A₁ ' is obtained by inverting thecode B₂ ' in the inverter 42₂ and adding it to the delayed code A₂ 'dafter amplifying it by a factor K₂ in the amplifier 46₂. Thus equations(14) and (15) are implemented.

The base codes a and b are obtained utilizing a third compression stage36₃ which is identical to the preceding stages with the exception of thetime delay provided by the time delays 48₁ and 48₂ which respectivelydouble at each stage to a final delay of 2^(p-1) ×τ where p is thenumber of compression stages. As shown, the A₁ ' code mate provided bythe second compression stage 36₂ is delayed by a time delay of 2τ in thedelay device 48₂ in order to line up the codes A₁ ' and B₁ '. As before,the code B₁ ' is amplified by a gain factor K₁ in the amplifier 38₃where it is added to the delayed code A₁ 'd in the adder 40₃ to yieldthe code A_(o) ' in accordance with equation (20). Code B_(o) ' isachieved by inverting the code B₁ ' by the inverter 42₃ and adding it tothe delayed code A₁ 'd after amplifying by a factor K₁ in the amplifier46₃. Codes A_(o) ' and B_(o) ' correspond to the original code matepairs a and b but now amplified by the gain factor (K₁ ² +1)(K₂ ² +1)(K₃² +1). In order to continue compressing the basic mate code pairsrequires that a third delay device 48₃ provide a time delay of 2^(p-1)×τ=4τ to make the two codes line up.

Although the pulse compressor shown in FIG. 3 has beed described forapparatus compressing a code length of 8n where n corresponds to thenumber of code bits a₁ . . . a_(n) and b₁ . . . b_(n) in each basic codemate a and b, the repeated application of the process described at eachcompression stage can compress a code structure of any length down tothe basic code mate pair. The compressed codes, moreover, are lobelessand are obtained with much less hardware than that required for otherknown approaches. For a code length comprising N blocks of n bits, onlyp compression stages are needed to compress the code down to n bitswhere 2^(p) =N. For example, with n=1 only p=10 compression stages witheach stage including one delay device, two amplifiers, two adders and aninverter would be required to compress a 1024 bit code down to alobeless impulse.

Thus what has been shown and described is the concept of compressingmultilevel code mate pairs that have been generated by interleaving twocode mates having autocorrelation functions of codes that are equal inmagnitude but of opposite sense for all values of time delay τ except atτ=0. These codes, moreover, can when appropriately utilized improve theentire field of communications since they are capable of optimizing alltransmission systems including not only communications systems, but alsomultiple access systems, radar systems, altimeters, fuses, missileguidance, navigation, traffic control, etc. by reducingself-interference, providing anti-jam (A/J) protection, low probabilityof intercept (LPI), as well as providing increased range and velocityresolution and their accuracy of measurement.

Having thus shown and described what is at present considered to be thepreferred method and means for implementing the subject invention, it isnoted that the same has been made by way of illustration and notlimitation. Accordingly, all modifications, alterations andsubstitutions may be made without departing from the spirit and scope ofthe invention as set forth in the appended claims.

I claim:
 1. A method of compressing a pair of multilevel expandeddigital noise codes comprised of first and second multilevel code matesin a plurality of successive stages where the expanded noise code arecomprised of a predetermined number of code bits defining the codelength of said code mates, comprising the steps of:at each stage,applying first and second multilevel expanded code mates; generatingthird and fourth multilevel code mates by respectively multiplying saidfirst and second code mates by a predetermined gain factor notnecessarily equalling unity; combining said first multilevel code matewith said fourth multilevel code mate in a first mathematical sense toform a first compressed code mate having one half the number of codebits of the preceding stage; combining said third multilevel code matewith said second multilevel code mate in a second mathematical sense toform a second compressed code mate also having one half the number ofcode bits of the preceding stage; and delaying said first compressedcode mate by a time delay equal to an integer multiple of the timeduration of a code bit prior to applying said first and secondcompressed code mates to the next stage whereby said compressed codemates become input code mates for the said next stage.
 2. The method ofclaim 1 wherein said time delay is doubled in each succeeding stage ofsaid plurality of successive stages.
 3. The method of claim 2 whereinsaid integer multiple of the first stage of said plurality of stages isunity.
 4. The method of claim 2 wherein said step of combining in afirst mathematical sense comprises the step of adding said first andfourth multilevel code mates.
 5. The method of claim 4 wherein said stepof combining in a second mathematical sense comprises the step ofsubtracting said second multilevel code mate from said third multilevelcode mate.
 6. The method of claim 4 wherein said step of combining in asecond mathematical sense comprises the step of generating thecomplement of said second multilevel code mate and adding it to saidthird code mate.
 7. The method of claim 2 wherein said pair ofmultilevel noise codes comprise expanded code mates formed byinterleaving code mate pairs.
 8. The method of claim 7 wherein said codemate pairs comprise equal length binary noise codes.
 9. The method ofclaim 8 wherein said code mate pairs comprise noise codes wherein thepair of compressed code mates of the last stage comprise an impulseautocorrelation function.
 10. The method of claim 1 and additionallyincluding the steps of:generating a pair of multilevel noise codescomprised of said first and second multilevel code mates; multiplexingsaid pair of multilevel noise codes; modulating said multiplexedexpanded noise codes on an RF carrier of communications apparatus andradiating said carrier; receiving and demodulating said carrier incommunications apparatus to provide a multiplexed coded received signal;demultiplexing said received signal to provide respective coded signalsof said pair of multilevel noise codes; and thereafter executing theabove recited compression steps.
 11. The method of claim 1 wherein eachstage comprises one of p successive stages which operate to compressN=2^(p) code bits down to a last pair of basic code mate signalsrespectively comprising n bit signals.
 12. Apparatus for compressing apair of expanded noise codes comprised of first and second multilevelcode mates and having a predetermined number of code bits defining thecode length of said code mates, comprising:a plurality of series coupledcompression stages, each stage including, means for generating third andfourth multilevel code mates respectively comprised of first and secondmultilevel input code mates multiplied by a predetermined gain factornot necessarily equalling unity; means for combining said firstmultilevel code mate with said fourth multilevel code mate in a firstmanner to form a first compressed code mate having one half the numberof code bits of the first and second multilevel input code mates; meansfor combining said third multilevel code mate with said secondmultilevel code mate in a second manner to form a second compressed codemate also having one half the number of code bits of the first andsecond input code mates; and means for delaying said first compressedcode mate by a time delay equal to an integer multiple of the code bitwidth of said codes for providing a first output code mate coincident intime with said second compressed code mate, said second compressed codemate being a second output code mate.
 13. The apparatus of claim 12 andwherein said means for combining in a first manner comprises means forlinearly adding said first multilevel code mate with said fourthmultilevel code mate.
 14. The apparatus of claim 13 wherein said meansfor combining in a second manner comprises means for linearlysubtracting said second multilevel code mate from said third multilevelcode mate.
 15. The apparatus of claim 13 wherein said means forcombining in a second manner comprises means for generating thecomplement of said second multilevel code mate, and means for linearlyadding said third multilevel code mate to said complement of said secondmultilevel code mate.
 16. The apparatus of claim 12 wherein said pair ofcode mates comprise equal length binary noise codes.
 17. The apparatusof claim 12 wherein said pair of multilevel noise codes comprise noisecodes producing impulse autocorrelation functions when compressed to asingle code bit.
 18. The apparatus of claim 12 wherein said pair ofmultilevel noise codes comprise expanded multibit code mates generatedby interleaving one code mate having a first amplitude with the othercode mate having a second amplitude.
 19. The apparatus of claim 12 andfurther comprising:means for generating said pair of expanded multilevelnoise codes; means for multiplexing said pair of multilevel noise codes;communications apparatus including means for modulating said multiplexedexpanded noise codes on an RF carrier and radiating said carrier;communications apparatus including means for receiving and demodulatingsaid carrier to provide a multiplexed coded receive signal; means fordemultiplexing said received signal to provide respective coded signalsof said pair of expanded noise codes, and means coupling saiddemultiplexed multilevel noise codes to the first of said plurality ofcompression stages.
 20. The apparatus of claim 19 wherein said means forgenerating said pair of expanded multilevel noise codes includes meansfor generating and interleaving multilevel code mate pairs to form saidfirst and second multilevel code mates.
 21. The apparatus of claim 12and additionally including linear adder means coupled to the final stageof said plurality of compression stages for combining said first andsecond compressed output code mates to provide for compression to asingle code bit output signal.